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wouldn't they have to do a hard fork like the did after the DAO hack?

"The exchange took full responsibility for this error and has ensured that there has been no impact on account funding. In a reddit post, the company's official account reported that "all withdrawals, including Ether, are being processed as per usual and client balances are unaffected."

The exchange caused their own problem. The DAO was based on the Ethereum protocol and was hacked. Two very different situations.